There are various processes that utilize radiation—e.g., sterilization, radiation therapy, food irradiation, quality checking, etc.—and these processes have a need to verify the radiation dose. Similarly, there is a large number of different methods to determine a dose—e.g., ion dosimetry (ionization in air), calorimetry (determination of heat in carbon or metals), thermoluminescence dosimetry (luminescence in solids), etc. The formation of radicals in solid organic substances on irradiation has been observed and the concentration of these radicals is proportional to the absorbed dose over a wide range. Those radical concentrations can be determined easily by means of electron spin resonance (ESR) spectroscopy. Amino acids—e.g., alanine—have been widely used for this purpose due to its availability and the relative simplicity of incorporating it into practical dosimeters. An advantage of the use of organic materials such as alanine over inorganic dosimeter systems is that it can be assumed that the irradiation-induced changes in organic materials are closer to radiation effects in living tissues.
Amino acid dosimetry is an accepted method to determine the irradiation dose of different irradiation processes. On irradiating with ionizing radiation, radicals will be produced in amino acids like alanine which are stable for long periods. This is mainly due to the inhibition of radical-radical recombinations in the crystalline structure of the material that prevents the migration of large molecule fragments. The non-destructive evaluation of the radical concentration can be done using ESR spectroscopy. The determination of irradiation doses by means of ESR techniques requires a sensitive, robust and reliable instrument that can be served by a laboratory worker. A useful instrument provides such features as automated procedures for calibration and measurements. Careful adjustment of the ESR spectrometer and the selection of suitable dosimeters allows the determination of dose rates in a range from 2 Gy to 200 kGy with a total uncertainty of 3.5% (confidence level of 95%). Amino acid dosimeters are small, stable, and easy to handle. They are characterized by their large measuring range and a low sensitivity to temperature and humidity. This allows for their application in radiation therapy, the irradiation of blood, as well as in industrial facilities for irradiation. The dosimeter system can be used for reference and routine dosimetry due to its high quality and low costs.
Alanine dosimeters are well known in the art. For example, in the reference: T. Kojima et al., “Alanine Dosimeters Using Polymers As Binders”, Applied Radiation & Isotopes, vol. 37, No. 6, (1986), Pergamon Journals Ltd., pp. 517-520, there are numerous references to dosimeters made in pellet, rod, and film formats. Dosimeters have been made both by industrial laboratories and at academic institutions. Most of these dosimeters are in the form of molded pellets or rods. The alanine is generally blended with a synthetic or natural rubber, compounded and molded under pressure to form a variety of shapes (U.S. Pat. No. 4,668,714, J.P. 203276 J.P. 0125085, J61057-878-8). There are also references in the literature to extruded films (J01102-388-A). These extruded products, while working well, have several deficiencies. Their manufacture often requires the use of high pressures and temperatures during the molding process requiring molding equipment that limits the sizes and shapes available. Molded dosimeters are also limited in that only moldable polymeric binders may be used. The use of molded dosimeters is also somewhat restrictive, as the size of the dosimeters tends to be very small, leading to difficulties in handling and possibly loss during irradiation.
A potential solution to these difficulties would be an amino acid dosimeter coated onto a flexible support wherein the support serves not only to hold the amino acid, but also provides the user with a length and width that allow easy handling. Such a coated dosimeter has been described in DE19637471 A. In this art, the alanine is coated from two, specific binders—a polyoctenamer or polystyrene. Both of these binders are brittle materials and made the coating of thick alanine layers with good mechanical properties very difficult, especially when the thickness of the dosimeter layer is >100 microns. The ability to bend and shape the amino acid dosimeter coated on to the plastic support can be very important in some applications, and is a significant limitation of the coated dosimeters described in the art.
The response of an amino acid dosimeter to ionizing radiation is proportional to the amount of amino acid coated on the dosimeter. While within a given manufacturing batch, the coated coverage may be very uniform, batch-to-batch variation makes it very important that dosimeters from a given batch be identifiable so calibration standards can be developed and used. Placing the lot number identification directly on the dosimeter is an excellent way to allow traceability back to the calibrations standard.
It would be useful to have a method of measuring an absorbed dose of ionizing radiation using a measuring device that is flexible and durable and that bears an integral identification mark.